Spin-transfer torque is an effect in which the orientation of a magnetic layer may be modified using a spin-polarized current.
Charge carriers, such as electrons, have a relatively small quantity of intrinsic angular momentum, referred to as spin. An un-polarized electrical current has approximately 50% spin-up electrons and 50% spin-down electrons. A spin-polarized current has more spin-up or spin-down electrons. By passing a current through a magnetic layer, one can produce a spin-polarized current. When a spin-polarized current is directed into a magnetic layer, angular momentum may be transferred to the magnetic layer, which may induce or excite oscillations in the magnetic layer. Above a threshold, the oscillations may trigger a switch in magnetization orientation of the layer. Such effects may occur in nanometer scale devices.
A magnetic tunnel junction (MTJ) may include an oxide layer sandwiched between first and second ferromagnetic (FM) layers, also referred to as a reference layer and a free layer, respectively. The reference layer may have a fixed magnetic orientation or state, and the free layer may have a variable magnetic orientation or state. The MTJ exhibits relatively low resistance when the magnetic alignment the reference and free layers is parallel, and relatively high resistance when the magnetizations are counter-parallel.
A MTJ may be implemented as a non-volatile memory cell in which digital logic values of 0 and 1 are associated with corresponding magnetization alignment states of the MTJ. Multiple MTJ memory cells may be implemented as a spin-transfer torque random-access memory (STT-RAM).
A logic value may be written to a MTJ memory cell by directing a write current through the MTJ sufficient to overwrite, or flip an existing magnetization alignment of the MTJ.
A logic valued stored in a MTJ memory cell may be read based on resistance of the memory cell, which may be determined based on a read current that passes through the memory cell.
The read current, or read current density, should be less than a critical current or critical current density that would re-orientate or overwrite the existing magnetization alignment of the MTJ.
Nevertheless, even a small read current may assert a torque on the magnetization orientation of an MTJ. In addition, a MTJ may be subject to random thermally induced torques, which may vary with process, voltage, and/or temperature variations. The sum of all these torques may result in switching or flipping of the magnetization orientation of the MTJ. This may be exasperated where higher read currents are employed to decrease read times.
A write back scheme may be employed after a read operation to ensure that the MTJ has the intended magnetization state. However, if the read operation causes the magnetization orientation of the MTJ to switch or flip early in a read operation, the read operation may read or sense the incorrect flipped state of the MTJ.
In the drawings, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears.